Lithology classification variable T2 cut-off value-based effective porosity calculation method and equipment

Abstract

The invention belongs to the technical field of oil and gas geology and exploration and development engineering, and discloses an effective porosity calculation method and equipment based on a lithologic classification variable T2 cutoff value. The method comprises the following steps: determining lithology, mineral composition and content, pore structure, total porosity and effective porosity of a rock core through a rock core experiment; the total porosity of nuclear magnetic logging is calibrated through the total porosity, and the core well depth is subjected to homing correction; using lithology, mineral composition and content scale elements for logging, calculating and constructing a lithology indication curve and identifying a lithology category; nuclear magnetic logging is carried out through the effective porosity scale of the rock core, and the effective porosity T2 cut-off value of the rock core is calculated; carrying out regression analysis on the rock core lithology indication curve value and the rock core effective porosity T2 cut-off value, and determining a variable T2 cut-off value fitting relational expression of each lithology; and calculating the effective porosity of the target interval. According to the method, the calculation precision of the effective porosity of unconventional oil and gas reservoirs with complex lithology and high heterogeneity, such as continental facies mixed shale, can be effectively improved.

Description

Technical Field

[0001] The invention belongs to the technical field of oil and gas geology and exploration and development engineering, and in particular relates to an effective porosity calculation method and equipment based on lithology classification variable T2 cutoff value. Background Art

[0002] The effective porosity of rock is the core parameter for describing rock physical properties. At present, it is a common practice in the industry to calculate effective porosity using the nuclear magnetic resonance logging T2 cutoff value. However, for unconventional oil and gas reservoirs with variable lithology and complex pore structure, such as mixed shale oil reservoirs in continental basins, the commonly used logging porosity calculation method is often difficult to evaluate effective porosity. There are three main methods for calculating effective porosity using the nuclear magnetic resonance T2 cutoff method at home and abroad:

[0003] (1) Fixed cutoff value method. The 3ms used in the calculation of effective nuclear magnetic resonance porosity at home and abroad is used as the effective porosity cutoff value of sandstone for calculation; in addition, there is a similar method of classifying reservoirs according to the actual situation, such as sedimentary environment, mud content, etc., and then using different fixed cutoff values ​​for porosity calculation. The advantage of this method is that the calculation is simple and convenient, and it has sufficient calculation accuracy for reservoirs with stable lithology and simple pore structure. The disadvantage is that due to the variable lithology, complex pore structure, and strong heterogeneity of reservoirs, they often do not have a unified T2 cutoff value. At this time, the fixed cutoff value method will lose accuracy;

[0004] (2) Experimental analysis method. Using industry standards (such as the national standard GB / T 29172-2012) or experimental methods developed for rocks with certain characteristics (such as patent documents CN110646331A, CN111175207A), it is often possible to obtain effective porosity experimental values ​​with high accuracy; however, the cost of obtaining cores is high, and it is impossible to obtain a considerable number of cores for each well. Therefore, the experimental analysis method cannot perform continuous effective porosity characterization in the vertical direction;

[0005] (3) Pore structure parameter method. This method uses the capillary pressure curve, nuclear magnetic permeability, T2 geometric mean and other parameters that can reflect the pore structure to calibrate the T2 cutoff value, and constructs the conversion relationship between the T2 cutoff value and the pore structure parameter to achieve the calculation of the vertically continuously variable T2 cutoff value. The advantage of this method is that it saves a lot of coring and experimental costs. The disadvantage is that in layers with complex lithology and strong heterogeneity, affected by multiple factors such as diagenesis, oil content, wettability, etc., the experimental data change law is complex, and it is difficult to obtain a high-precision model.

[0006] Chinese patent CN111650108A discloses a method and device for determining the effective porosity of shale rock. The method for determining the effective porosity of shale rock includes: obtaining the total volume of the shale rock to be measured, the skeleton volume of the shale rock to be measured, and the volume of particles and / or dust in the shale rock to be measured; determining the volume of a hygroscopic medium; correcting the skeleton volume using the volume of the hygroscopic medium to obtain a skeleton-corrected volume; and obtaining the effective porosity of the shale rock to be measured according to the total volume, the skeleton-corrected volume, and the volume of particles and / or dust. However, the technical solution of this patent solves the problem of inaccurate measurement of effective porosity of mudstone (mudstone has a high clay mineral content and belongs to clay shale. The main controlling factor of effective porosity is the clay mineral content. Montmorillonite among clay minerals is particularly easy to absorb water and swell, which leads to core rupture, and thus makes the effective porosity measurement inaccurate). It is not suitable for unconventional oil and gas reservoirs such as continental mixed shale with variable lithology, complex pore structure and strong heterogeneity (continental mixed shale has a low clay mineral content, and the main controlling factor of effective porosity is not clay minerals, but two major types of rock-forming minerals, felsic and carbonate rocks); at the same time, the technical solution of this patent can only obtain discrete core data through experimental measurement, and cannot obtain continuous logging evaluation results. Summary of the invention

[0007] The present invention aims to solve at least one technical problem existing in the prior art or related technology, and provides a method and device for calculating effective porosity based on lithology classification variable T2 cutoff value, which can effectively solve the problem of low accuracy of effective porosity calculation.

[0008] In order to achieve the above technical objectives, the present invention adopts the following technical solutions:

[0009] A method for calculating effective porosity of rocks with a variable T2 cutoff value based on lithology classification, the method comprising the following steps:

[0010] Step S1: obtaining a core of the target layer, and performing thin section observation, X-ray diffraction, water-saturated nuclear magnetic resonance experiment and helium injection porosity experiment on the core to clarify the lithology, mineral composition and content, pore structure, total porosity and effective porosity of the core;

[0011] Step S2: using the core total porosity calibration nuclear magnetic logging total porosity in step S1, the well depth position of the core is calibrated;

[0012] Step S3: using the core lithology, mineral composition and content calibration element logging in step S1, calculate and obtain the content curve of each mineral in the target layer; construct a lithology indicator curve based on the content curve of each mineral, and then identify the major lithology categories;

[0013] Step S4: using the core effective porosity calibration nuclear magnetic logging in step S1, back-calculate the core effective porosity T2 cutoff value;

[0014] Step S5: extract the lithology indicator curve value corresponding to the core of each lithology according to different lithologies; then perform regression analysis on the lithology indicator curve value corresponding to the core and the effective porosity T2 cutoff value of the core to determine the fitting relationship of the variable T2 cutoff value of each lithology;

[0015] Step S6: Based on the lithology classification of the target layer, the variable T2 cutoff value fitting relationship constructed in step S5 is used to calculate the logging effective porosity T2 cutoff value of each depth point of the target layer according to the lithology indicator curve value of the corresponding lithology of the target layer; and according to the logging effective porosity T2 cutoff value of each depth point of the target layer, the effective porosity corresponding to each depth point of the target layer is calculated.

[0016] Furthermore, in step S3, the lithology indicator curve can continuously characterize the vertical lithology changes of the formation, and is a function with the content of each mineral in the rock as the independent variable.

[0017] Furthermore, in step S3, the content of each mineral is calculated by element logging.

[0018] Furthermore, the element logging includes ECS logging, FEM logging, PNX logging or lithology scanning logging.

[0019] Furthermore, in step S3, the calculation formula of the lithology indicator curve is:

[0020] LIC=f(V1,V2,…,V n )

[0021] In the above formula, LIC is the lithology indicator curve, dimensionless; V1, V2, ..., V n It is the content of the first, second,…, nth mineral, %.

[0022] Further, in step S5, the variable T2 cutoff value fitting relationship is as follows:

[0023] T 2cutoff_i =g i (LIC)

[0024] In the above formula, T 2cutoff_i is the variable T2 cutoff value of the i-th lithology, ms; g i is the T2 cutoff value fitting relationship of the ith lithology, ms.

[0025] Furthermore, in step S6, according to the logging effective porosity T2 cutoff value of each depth point of the target layer, the calculation formula for calculating the effective porosity corresponding to each depth point of the target layer is:

[0026]

[0027] In the above formula, φ e_k is the effective porosity of the kth depth point in the target layer, a decimal; P j is the porosity component with relaxation time j corresponding to the kth depth point of the target layer, a decimal; T 2cutoff_k is the T2 cutoff value of the k-th depth point of the target layer, ms; T2 is the starting time of the effective porosity of the k-th depth point of the target layer, ms.

[0028] At the same time, the present invention provides a computer-readable storage medium, wherein the computer-readable storage medium stores a computer program, wherein the computer program includes program instructions, and when the program instructions are executed by a computer, the computer executes any of the methods described above.

[0029] In addition, the present invention also provides an electronic device, which includes a processor, a memory, and a computer program stored in the memory and executable on the processor, and when the processor executes the program, the method described in any one of the above items is implemented.

[0030] Compared with the prior art, the beneficial effects of the present invention are:

[0031] (1) Compared with the fixed cutoff value method, the fixed cutoff value method does not take into account the changes caused by factors such as lithology and mineral changes, and is therefore only applicable to clastic rock formations with single lithology and stability; while the rock effective porosity calculation method of the present invention based on the variable T2 cutoff value of lithology classification fully takes into account the influence of changes in lithology and mineral content on the nuclear magnetic T2 cutoff value, and can be better applied to the calculation of effective porosity in formations with complex lithology, fast changes in mineral content, etc.;

[0032] (2) Compared with the experimental analysis method, the rock effective porosity calculation method based on the variable T2 cutoff value of lithology classification of the present invention can be promoted and applied in a certain exploration area after core calibration, without spending a lot of money on core experimental measurement, thus saving measurement cost and measurement cycle;

[0033] (3) Compared with the pore structure parameter method, the pore structure parameter method often cannot obtain a high-precision model in layers with complex lithology and strong heterogeneity; the method of the present invention calculates the effective porosity of rocks based on the variable T2 cutoff value of lithology classification, and does not start from the idea of ​​pore structure evaluation, so there is no need to consider the above problems;

[0034] (4) Compared with the technical solution of patent CN111650108A, the rock effective porosity calculation method based on the variable T2 cutoff value of lithology classification of the present invention can be applied to unconventional oil and gas reservoirs such as continental mixed shale with variable lithology, complex pore structure and strong heterogeneity. At the same time, the method of the present invention can calibrate the core experimental data and calculate and obtain continuous logging evaluation results. After the core calibration of a certain exploration area, it can be promoted and applied in the exploration area, and has stronger versatility and practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 A schematic diagram of a method flow of an embodiment of the present invention;

[0036] Figure 2 Figure 1 is a relationship diagram between the lithology indicator curve values ​​of different lithologies and the core nuclear magnetic T2 cutoff value in the embodiment of the present invention, wherein Figure (a) is dolomitic siltstone, Figure (b) is gray siltstone, Figure (c) is argillaceous siltstone, Figure (d) is dolomitic mudstone, Figure (e) is gray mudstone, Figure (f) is silty mudstone, and Figure (g) is argillaceous dolomite;

[0037] Figure 3 This is a diagram showing the calculation results of the effective porosity of a certain well in an embodiment of the present invention. DETAILED DESCRIPTION

[0038] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] Example 1

[0040] The embodiment of the present invention provides a method for calculating effective porosity of rocks with a variable T2 cutoff value based on lithology classification. Taking a well in a reservoir as an example, combined with Figure 1 As shown, the method comprises the following steps:

[0041] Step S1: Obtain the core of the target layer, and perform thin section observation, X-ray diffraction (XRD), water-saturated nuclear magnetic resonance experiment and helium injection porosity experiment on the core to clarify the main properties of the core, such as lithology, mineral composition and content, pore structure, total porosity and effective porosity.

[0042] Step S2: Using the core total porosity calibration nuclear magnetic logging total porosity in step S1, a fine repositioning correction is performed on the core position depth to ensure the accuracy of the subsequently constructed model and the accuracy of the final calculation results.

[0043] Step S3: Use the core lithology, mineral composition and content information in step S1 to calibrate lithology scanning logging, calculate and obtain the content curve of each mineral in the target layer; construct a lithology indicator curve based on the content curve of each mineral, and then identify the major lithology categories.

[0044] Among them, the lithology indicator curve (LIC) is a function of the mineral content, and there are many specific construction formulas. In the embodiment of the present invention, the construction formula of LIC is:

[0045]

[0046] In the above formula (1), LIC is the lithology indicator curve, dimensionless; V is the volume fraction of each mineral, %.

[0047] Step S4: using the core effective porosity calibration nuclear magnetic logging in step S1, back-calculate the core effective porosity T2 cutoff value.

[0048] Step S5: Based on the lithology indicator curve and lithology classification identification results constructed in step S3, extract the lithology indicator curve value corresponding to the core of each lithology according to different lithologies; then perform regression analysis on the lithology indicator curve value corresponding to the core and the effective porosity T2 cutoff value of the core to determine the fitting relationship of the variable T2 cutoff value of each lithology.

[0049] In the embodiment of the present invention, a total of 7 major lithologies are identified, and the following are constructed for different lithologies: Figure 2 The relationship between the lithology indicator curve value and the core effective porosity T2 cutoff value is shown, where (a) is dolomitic siltstone, (b) is gray siltstone, (c) is argillaceous siltstone, (d) is dolomitic mudstone, (e) is gray mudstone, (f) is silty mudstone, and (g) is argillaceous dolomite; the abscissa in each figure is the lithology indicator curve value, dimensionless; the ordinate is the core effective porosity T2 cutoff value, ms.

[0050] from Figure 2 It can be seen that the correlation between the fitting equations of various lithologies is good. Specifically, the fitting equation of each lithology is:

[0051] Dolomitic siltstone:

[0052] y=34102e -3.851x , R 2 =0.6810(2)

[0053] Gray siltstone:

[0054] y=421.98e -1.928x , R 2 =0.8334(3)

[0055] Silty mudstone:

[0056] y=2.3335e -0.526x , R 2 =0.7963(4)

[0057] Dolomitic mudstone:

[0058] y=10567e -3.643x , R 2 =0.6848(5)

[0059] Limestone:

[0060] y=2765.5e -4.006x , R 2 =0.7906(6)

[0061] Muddy siltstone:

[0062] y=7.7251e -0.255x , R 2 =0.6432(7)

[0063] Argillaceous dolomite:

[0064] y=835.95e -4.352x , R 2 =0.6378(8)

[0065] In the above formulas (2)-(8), y is the T2 cutoff value of the core effective porosity, in ms; x is the lithology indicator curve value, dimensionless; and R is the correlation coefficient, dimensionless.

[0066] Step S6: For the seven different lithologies, the variable T2 cutoff value fitting relationship constructed in step S5 is used respectively to calculate the logging effective porosity T2 cutoff value of each depth point of the target layer according to the lithology indicator curve value of the corresponding lithology of the target layer; and according to the logging effective porosity T2 cutoff value of each depth point of the target layer, the effective porosity corresponding to each depth point of the target layer is calculated.

[0067] Among them, according to the logging effective porosity T2 cutoff value of each depth point of the target layer, the calculation formula for calculating the effective porosity corresponding to each depth point of the target layer is:

[0068]

[0069] In the above formula (9), is the effective porosity of the kth depth point in the target layer, a decimal; P j is the porosity component with relaxation time j corresponding to the kth depth point of the target layer, a decimal; T 2cutoff_kis the T2 cutoff value of the k-th depth point of the target layer, ms; T2 is the starting time of the effective porosity of the k-th depth point of the target layer, ms.

[0070] Figure 3 This is the final effective porosity evaluation result diagram of the embodiment of the present invention, where the first track from left to right is the depth track, tracks 2-4 are conventional curve tracks, track 5 is the mineral content track calculated by lithology scanning logging; track 6 is the lithology classification based on mineral content; track 7 is the lithology indicator curve calculated by mineral content. It can be observed that whether it is mineral content, logging lithology classification or lithology indicator curve, it shows that the lithology of the well changes very rapidly.

[0071] at the same time, Figure 3 Track 8 is the variable T2 cutoff value calculated by lithology; Tracks 9, 10, and 11 are the effective porosities calculated using the fixed cutoff value of 3ms, the fixed cutoff value for different lithology categories, and the variable T2 cutoff value for different lithology in the embodiment of the present invention, respectively. The points in Tracks 9-11 are the effective porosities obtained from the core experimental analysis. It can be seen that the huge variation range of the T2 cutoff value indicates that it is unreasonable to calculate the effective porosity using the fixed cutoff value. The calculation result of the effective porosity using the variable T2 cutoff value in the embodiment of the present invention is most consistent with the effective porosity of the core, with a relative error of only 8%, which meets the accuracy requirements of geological related research on well logging physical property evaluation.

[0072] Example 2

[0073] An embodiment of the present invention provides a computer-readable storage medium, wherein the computer-readable storage medium stores a computer program, wherein the computer program includes program instructions, and when the program instructions are executed by a computer, the computer executes the method described in Embodiment 1.

[0074] Example 3

[0075] An embodiment of the present invention provides an electronic device, which includes a processor, a memory, and a computer program stored in the memory and executable on the processor. When the processor executes the program, the method described in Embodiment 1 is implemented.

[0076] The above description is only an embodiment of the present invention and is not intended to limit the present invention. Any modification, equivalent replacement and improvement made within the application scope of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for calculating effective porosity of rocks with a variable T2 cutoff value based on lithology classification, characterized in that: The method comprises the following steps: Step S1: obtaining a core of the target layer, and performing thin section observation, X-ray diffraction, water-saturated nuclear magnetic resonance experiment and helium injection porosity experiment on the core to clarify the lithology, mineral composition and content, pore structure, total porosity and effective porosity of the core; Step S2: using the core total porosity calibration nuclear magnetic logging total porosity in step S1, the well depth position of the core is calibrated; Step S3: using the core lithology, mineral composition and content calibration element logging in step S1, calculate and obtain the content curve of each mineral in the target layer; construct a lithology indicator curve based on the content curve of each mineral, and then identify the major lithology categories; Step S4: using the core effective porosity calibration nuclear magnetic logging in step S1, back-calculate the core effective porosity T2 cutoff value; Step S5: extract the lithology indicator curve value corresponding to the core of each lithology according to different lithologies; then perform regression analysis on the lithology indicator curve value corresponding to the core and the effective porosity T2 cutoff value of the core to determine the fitting relationship of the variable T2 cutoff value of each lithology; Step S6: Based on the lithology classification of the target layer, the variable T2 cutoff value fitting relationship constructed in step S5 is used to calculate the logging effective porosity T2 cutoff value of each depth point of the target layer according to the lithology indicator curve value of the corresponding lithology of the target layer; and according to the logging effective porosity T2 cutoff value of each depth point of the target layer, the effective porosity corresponding to each depth point of the target layer is calculated.

2. The method according to claim 1, characterized in that In step S3, the lithology indicator curve can continuously characterize the vertical lithology changes of the formation, and is a function with the content of each mineral in the rock as the independent variable.

3. The method according to claim 2, characterized in that In step S3, the content of each mineral is calculated by element logging.

4. The method according to claim 3, characterized in that The element logging includes ECS logging, FEM logging, PNX logging or lithology scanning logging.

5. The method according to any one of claims 1 to 4, characterized in that: In step S3, the calculation formula of the lithology indicator curve is: LIC=f(V1,V2,…,V n ) In the above formula, LIC is the lithology indicator curve, dimensionless; V1, V2, ..., V n It is the content of the first, second,…, nth mineral, %.

6. The method according to claim 1, characterized in that In step S5, the variable T2 cutoff value fitting relationship is as follows: T 2cutoff_i =g i (LIC) In the above formula, T 2cutoff_i is the variable T2 cutoff value of the i-th lithology, ms; g i is the T2 cutoff value fitting relationship of the ith lithology, ms.

7. The method according to claim 6, characterized in that In step S6, according to the logging effective porosity T2 cutoff value of each depth point of the target layer, the calculation formula for calculating the effective porosity corresponding to each depth point of the target layer is: In the above formula, φ e_k is the effective porosity of the kth depth point in the target layer, a decimal; P j is the porosity component with relaxation time j corresponding to the k-th depth point of the target layer, a decimal; T 2cutoff_k is the T2 cutoff value of the k-th depth point of the target layer, ms; T2 is the starting time of the effective porosity of the k-th depth point of the target layer, ms.

8. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores a computer program, wherein the computer program includes program instructions, and when the program instructions are executed by a computer, the computer executes the method according to any one of claims 1 to 7.

9. An electronic device comprising a processor, a memory, and a computer program stored in the memory and executable on the processor, wherein: When the processor executes the program, the method according to any one of claims 1 to 7 is implemented.

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